1. Field of the Invention
The present invention relates to color conversion using a table.
2. Description of the Related Art
Today, with development of computers, digital image data is very commonly distributed. Color matching is known as a technique of converting input image data to color signals suitable for devices, which are to be used, by taking differences in color representation among the devices into account.
A conventional color matching method is described below by referring to a conceptual diagram illustrated in FIG. 21. Input data (RGB-data) is converted by an input profile 101 into XYZ data in a device-independent color space. Because an output device cannot reproduce colors that are outside a color gamut of the output device, gamut mapping is performed on the XYZ data, into which the input data has been converted, to map the XYZ data onto color data which is within the color gamut of the output device. The data, on which the gamut mapping is performed, is converted from the device-independent color space into CMYK data of an output-device-dependent color space.
In color matching, a reference white point and ambient light are fixed. For example, according to a profile specified by the International Color Consortium (ICC), Profile Connection Space (PCS) includes XYZ values or Lab values based on D50 reference. Therefore, correct color reproduction is ensured when an input original document and a printout are viewed under an illuminant of D50 reference. Under an illuminant based on another reference, correct color reproduction is not assured.
Japanese Patent Application Laid-Open No. 2002-281338 discusses a method enabled to perform good color matching, regardless of viewing conditions, such as ambient light, to solve the problem of the conventional color matching.
In the method discussed in Japanese Patent Application Laid-Open No. 2002-281338, converting steps are explicitly separated from one another, as compared with the conventional color matching method described by referring to FIG. 21. Each converting step can easily be updated to perform an optimum conversion process for devices.
The method discussed in Japanese Patent Application Laid-Open No. 2002-281338 can flexibly be applied to a wide range of systems, such as a scanner/printer system and a monitor/printer system, by a color management system. For example, in a desktop publishing (DTP) system, a color image displayed on the screen of a monitor can be matched in appearance with an image output to a printer by designating a monitor profile and a printer profile as a source profile and a destination profile, respectively.
Because quality of appearance match depends upon the profiles, it is important to create a profile that accurately associates a device-dependent color with a color in a PCS space. To that end, color estimation, which is estimation of a physical stimulus value obtained when a device-dependent color is actually printed or displayed, should accurately be performed.
A conventional color estimation technique is, for example, what is called a table-type color estimation technique using patch colorimetry values and interpolation. Various interpolation methods, such as a tetrahedron interpolation method, a cube interpolation method, and a triangular prism interpolation method, have been proposed as linear interpolation methods. Diverse interpolation methods, such as a spline interpolation method, have been proposed as nonlinear interpolation methods.
For example, Japanese Patent Application Laid-Open No. 10-136216 has proposed a method of changing an interpolation technique and a grid point interval according to whether input color data belongs to a specific unit cube when color estimation is performed.
The method discussed in Japanese Patent Application Laid-Open No. 10-136216 can enhance accuracy of color conversion of important colors by setting one or more specific unit cubes including important colors, for instance, a flesh, and an azure. Also, this method can enhance color reproducibility of a gray by setting a plurality of specific unit cubes including achromatic colors.
Japanese Patent Application Laid-Open No. 2000-261684 has proposed a method of performing, in a case where input color components are YMCK and output color components are Y′M′C′K′, color estimation by performing a nonlinear interpolation when a color component Y′ is calculated, and also performing a linear interpolation when each of the other color components is calculated.
However, the method discussed in Japanese Patent Application Laid-Open No. 10-136216 has a problem in that discontinuity may occur on a boundary surface between a specific unit cube and the outside thereof and may adversely affect tonability. In a case where input values are RGB values represented by digital signals and have 256 levels respectively corresponding to, for example, integer values ranging from 0 to 255, the input values are integer values, that is, discrete values. Thus, the discontinuity on the boundary surface is unlikely to become problematic. However, in a case where the input values are floating point values, tonability is highly likely to deteriorate.
In a case where interpolation methods respectively applied to a specific unit cube and the outside thereof are different, especially, where the interpolation methods, such as a linear interpolation method and a nonlinear interpolation method, are largely different in properties, results of interpolations respectively obtained by the two interpolation methods from the input values differ from each other. That is, even when the input values represent data on the boundary surface, results of the interpolations respectively obtained by the two interpolation methods differ from each other. Thus, it is apparent that the discontinuity occurs on the boundary surface. Similarly, even when the grid point interval is changed, and when the same interpolation method is applied to both cases, in an event where an actually measured value at a grid point in one of the cases differs from a result of interpolation in the other case, discontinuity naturally occurs.
The method discussed in Japanese Patent Application Laid-Open No. 2000-261684 is effective in a case where an input color space and an output color space are of the same kind. However, in a case where color estimation is performed when a device-dependent color is converted into a color in a PCS space, for example, in a case where color estimation is performed at conversion from an RGB space into a Lab space, an input color space and an output color space are of different kinds. Thus, this method cannot successfully be applied to this case.